I use firewood rounds as back stops as well. They can absorb an amazing amount of lead. I haven't found the limit yet. I have three at different distances. Just pin paper or cardboard targets right to them. Around here they don't cost anything. Trees everywhere.
Just be careful using wood you don't drill through it with repeated shots in the same spot. I was using a chunk of pine 8x8 scrap from a project to break in my Edgun barrel. I don't recall how many shots it took but, there was a pretty deep hole in the wood (~3") and one really big glob of lead! LOL! From the picture posted here, that tree chunk looks PLENTY thick! Happy Shooting